The fetoplacental unit plays an essential role in the complicated hormonal interplay that is necessary for a successful pregnancy. Human choriogonadotropin (hCG), a hormone secreted by the trophoblast cells of the placenta, is present in the blood and urine during pregnancy. The primitive trophoblast begins producing hCG soon after fertilization and concentrations of hCG rise to peak values at 8-12 weeks of pregnancy, then decrease to a plateau level that is maintained during the remainder of the pregnancy. hCG may also be produced by certain trophoblastic or ectopic neoplasms.
Following fertilization, hCG stimulates the sustained secretion of progesterone by the corpus luteum, which is necessary for the growth of the uterine endometrium. hCG also is a stimulus for the production of fetal testicular androgens.
Decreased production of hCG can result in spontaneous abortion. Many abnormal pregnancies are associated with decreased levels of hCG as compared to normal pregnancies of the same duration. Most of these pregnancies end in spontaneous abortions. (B. Saxena, in Endocrinology of Pregnancy, Fuchs et al. Eds, Harper & Row, Philadelphia (1983) pp. 50-72).
The decidualized endometrium of the uterus also produces substances necessary for gestation. The decidualized endometrium and the placental trophoblast are contiguous tissues, and prior studies have investigated the possibility that each may influence the function of the other. Studies performed over a decade ago showed that hCG secretion from placental explants rose over the initial 72 hour period, leading researchers to suggest that loss of an endogenous hCG inhibitory factor (lost as it was removed from culture during changes of growth media) was responsible. (A. Golander et al., Endocrinology 102:597-605 (1978)). Yuen and coworkers have suggested that prolactin released by the decidua inhibits hCG production by trophoblasts. (B. H. Yuen, et al. Am. J. Obstet. Gynecol. 154:336-340 (1986)). Conversely, trophoblast hCG secretion has been found to be stimulated by interleukin-1 produced by placental phagocytic cells and decidual cells. (A. Flynn, et al., Science, 218:475-476 (1982) and S. Yagel et al., J. Clin. Endocrinol. Metab., 68:992-995 (1989)).
Recently, Vicovac and Genvacev provided evidence, through co-incubation of trophoblasts and decidual cells, that the decidua has an inhibitory effect on the synthesis of hCG and total protein by trophoblast cells. (L. J. Vicovac et al., Placenta 9:109-115, (1988)). They concluded that some endometrial secretory product or products act to inhibit total protein synthesis and hCG secretion and/or synthesis by the trophoblast tissue.
Characterization of an endometrial hCG inhibitor was heretofore unknown. Potential uses for such a substance include as an agent to induce therapeutic abortions and as a basis to develop methods of evaluating hCG inhibition in vivo. The present inventors noted the inhibitory effect of a substance produced by decidual cells, and searched for the active material. As a result, the present inventors have found that a protein of about 7,000 to about 10,000 daltons (as measured by ultrafiltration and gel chromatography) is produced by the decidua and has an inhibitory effect on hCG production by the trophoblast and by JEG-3 choriocarcinoma cells.